1. Field of the Invention
Embodiments of the present invention generally relate to catalyst deactivation in fluid catalytic cracking units. More particularly, embodiments of the present invention relate to apparatus and method for providing a sufficient amount of heat in a regenerator to satisfy the reactor heat demand when a light feedstock is being cracked that may not provide sufficient coke formation on the catalyst to fully satisfy the reactor heat demand.
2. Description of the Related Art
Fluid catalytic crackers (“FCC”) are a mainstay in the conversion of raw hydrocarbons into one or more preferred products. A typical FCC generally includes a few components: one or more riser reactors, one or more disengagers and one or more regenerators. A hydrocarbon feed and one or more catalysts are added to the riser reactor, which is maintained at an elevated temperature and/or pressure. The cracking of the hydrocarbons within the riser reactor produces one or more cracked hydrocarbons and small quantities of carbonaceous coke, which becomes deposited on the surface of the catalyst. The coke deposits deactivate the catalyst after passage through the riser reactor. After exiting the riser reactor, the cracked hydrocarbons and the coked catalyst are introduced to one or more disengagers where the coked catalyst is separated from the cracked hydrocarbons. The cracked hydrocarbons are then removed from the FCC for further processing and/or treatment.
The coked catalyst, on the other hand, is introduced to one or more regenerators where the coke is combusted, oxidized, and/or converted to one or more waste gases. This combustion process removes the coke from the surface of the catalyst, thereby regenerating the catalyst and permitting its recycle back to the riser reactor. At the same time, the combustion of the coke provides heat to increase the regenerator operating temperature so that the circulating catalyst supplies the reactor heat demand. The reactor heat demand consists mostly of the heat required to vaporize and heat the liquid feedstock to the desired reaction temperature as well as the heat of reaction from the hydrocarbon cracking process. Therefore, maintaining high temperatures throughout the FCC and ensuring sufficient coke build-up on the catalyst to provide the heat necessary to regenerate the catalyst are of primary importance in the operation of an FCC.
Light hydrocarbon feeds, those containing twelve or fewer carbon atoms, can be used as a feed to an FCC adapted to produce olefinic hydrocarbons such as ethylene and propylene. However, light hydrocarbon feeds generally produce insufficient coke deposition to support the reactor heat demand. The same can be said of highly hydrotreated and/or severely de-sulfurized feedstock that do not produce enough coke to close the heat balance.
Even at high severity conditions, the above-mentioned FCC units will typically require a supplemental fuel source to maintain catalyst regeneration temperatures. This is traditionally accomplished through the use of liquid oils, such as “torch oil” mixed with a small amount of steam. Although torch oil is generally injected directly into the regenerator fluid bed during start-up of the unit to bring the regenerator temperature up to the desired operating level, some applications employ torch oil continuously during normal operations to maintain a desired regenerator bed temperature. However, because the torch oil is typically injected directly into the oxidizing environment, it can generate extremely high temperatures in the vicinity of the flame and on the catalyst particles and the particles can become overwhelmed and saturated with the torch oil. As a result, the catalyst particles tend to overheat, resulting in an undue deactivation of the cracking catalyst.
There is a need, therefore, for an FCC process and system capable of processing a light feedstock or severely hydrotreated feedstocks that conventionally yield inadequate coke formation, yet improved somehow to satisfy the heat demand of the reaction system